This invention relates generally to improvements in processes for bleaching cellulosic materials in the form of pulp, such as wood pulp, and particularly to the rapid bleaching of high consistency pulp, with fewer washing stages, to obtain a pulp of improved quality and properties from a given bleach sequence.
Pulp, as it comes from the digester, whether produced from hardwood or softwood, contains residual coloring matter. While unbleached pulp may be used for the manufacture of certain grades of paper, for example, heavy wrapping paper and paper for use in bags, pulp which is to be used for printing or writing paper or paper which is to be dyed, must be bleached. Furthermore, bleaching may be required in order to remove impurities if the pulp is to be used as a raw material for the production of rayon, gun powder, and other cellulose products.
Depending upon the nature of the raw pulp and the end use of which the pulp will be employed, various chemical bleaching stages and various sequences of these stages have been used heretofore. Among the principal chemical bleaching stages which have been used are the chlorination stage (designated "C"), the caustic extraction stage (designated "E"), the hypochlorite stage (designated "H"), and the chlorine dioxide stage (designated "D"). In addition, both chlorine and chlorine dioxide may be used in the same stage (designated "C.sub.D ") or ("D.sub.C ") and the chemicals may be used as a mixture or added sequentially. Various combinations of the above stages have been employed depending upon the specific conditions and bleaching requirements. For example, common bleaching sequences may include the following: CEH, CEHD, CEHED, CEHDED, and CEDED. Of these, the C.sub.D EDED and C.sub.D EHDED are the recognized standard sequences for producing 88+GE Brightness market pulp.
The C.sub.D EDED sequence produces a high brightness pulp with a minimum of viscosity loss or cellulose degradation to the pulp. This results in a pulp which has high strength properties. The C.sub.D EHDED sequence also produces high brightness, but the hypochlorite stage causes degradation of the cellulose in a controlled fashion. This results in some loss in paper strength, but the pulp requires less mechanical beating in order to develop its maximum strength, compared with pulp bleached by the C.sub.D EDED sequence. The C.sub.D EHED sequence is used to make the same type of pulp as the C.sub.D EHDED sequence, but it has one fewer stage for control of brightness. The CEH sequence is used for semibleached pulp in the brightness range 65 to 75 GE Brightness Standard. The C.sub.D EHD sequence is normally not used for pulp requiring brightness greater than 86 GE because bleaching to higher brightness with this sequence generally results in a severe loss in viscosity and strength.
In the initial chlorination stage, chlorine is added to the washed pulp received from the digester. Ordinarily, the chlorination stage (C) is performed at temperatures in the range of about 30.degree. C. to 50.degree. C., with a pulp consistency of about 3 percent. Under these conditions, the reaction time in the chlorination tower is about 30 to 60 minutes. The chlorine reacts directly with the lignin and other impurities in the pulp. Chlorine dioxide may be used in conjunction with chlorine (C.sub.D) or in place of chlorine for the initial chlorination stage (D).
Following the chlorination stage, a caustic extraction stage (E) using a dilute aqueous solution of sodium hydroxide (0.5 to 5.0 percent NaOH based on oven-dry weight of pulp) is performed to dissolve the chlorinated and oxidized lignin as well as some of the resin. The extraction stage is usually performed at temperatures of about 50.degree. C. to 80.degree. C. for a period of about 60 to 120 minutes with a pulp consistency of 10 to 20 percent.
The next stage of bleaching is commonly a hypochlorite stage (H), although a chlorine dioxide stage is sometimes preferred. In the hypochlorite stage, either sodium hypochlorite (NaOCl) or calcium hypochlorite (Ca(OCl).sub.2) is used to further oxidize the remaining lignin and other impurities in the pulp. Some degradation of the pulp as a result of shortening the chain length of the cellulose molecule usually occurs in the hypochlorite stage. Normally, the hypochlorite stage is performed at temperatures between about 30.degree. C. and 50.degree. C., and at a pulp consistency of 3 to 15 percent. The time employed for the hypochlorite stage varies inversely with the pulp consistency and the temperature ranging from 1 to 2 hours at a 15 percent consistency at 30.degree. C., up to 5 hours at a 3 percent consistency. A third stage hypochlorite tower may be commonly operated at a temperature of about 35.degree. C. for a period of 90-120 minutes at a pulp consistency of 12 percent. There are a few hypochlorite stages in commercial operation where the temperature is as high as 80.degree. C., at 12 percent consistency, in which case the retention time can be as low as 5 minutes.
Following the hypochlorite stage, there may be a second alkaline or caustic extraction stage (E) or a chlorine dioxide stage (D). The chlorine dioxide stage is usually designed for a 3 to 5 hour operation at about 11 percent consistency and a temperature around 70.degree. C. to 80.degree. C. As chlorine dioxide is a relatively mild bleaching agent and will produce a good pulp over a fairly wide range of conditions, it is particularly effective late in a multi-stage operation since the high temperature will tend to soften shives and a residual of chlorine dioxide will bleach out the thus softened shives.
In addition to some combination of the various bleaching stages outlined above, it is conventionally considered necessary in the pulping and bleaching arts to provide a washing stage between each of the bleaching stages in the sequence in order to remove the spent bleaching agent and the products of chemical reaction from the pulp prior to the beginning of the next bleaching stage so that the chemical requirements of the bleaching process may be minimized. Washing is ordinarily carried out by diluting the pulp to low consistency (usually 0.5 to 1.25 percent) followed by thickening to 10 to 15 percent consistency (by removal of some water) and washing on a drum type washer wherein an excess of wash water displaces the liquid in the pulp. Thus, there are two types of washing occurring on drum washers, first a dilution wash in the washer vat and a displacement wash as shower water passes through the sheet on the drum.
The present practice of multistage bleaching thus requires a period of 12 to 18 hours to bleach pulp to the desired brightness and viscosity values. Moreover, it is apparent that both the temperature and the consistency or dilution of the pulp are varied from stage to stage and for the interstage washing operations necessitating a high consumption of water and steam. Furthermore, the long periods of time required for each bleaching stage introduce problems in the control of the pulp quality, especially during periods of varying production rates, since a long period of time must elapse from the time a change or adjustment in the operation is made until the effect of that change may be observed. Consequently, it is evident that if a significant reduction in bleaching time could be effected, improved control of the multistage bleaching process, as well as substantial savings in time, would result. Additionally, the adaptability of the process to advanced control techniques, using on-line sensors and process control computers, such as those disclosed in Histed et al. U.S. Pat. No. 4,013,506, granted Mar. 22, 1977, commonly assigned, would be enhanced.
With the need to decrease energy consumption and water pollution from bleacheries, there has been a trend towards reduced fresh water consumption in bleacheries by means of countercurrent washing. See in this regard, U.S. Pat. No. 3,698,995; U.S. Pat. No. 4,104,114; Histed and Nicolle, Tappi, Vol. 59, No. 3, pp. 75-77 (March 1976); and Nelson et al., Tappi, Vol. 55, No. 6, pp. 933-936 (June 1972). These proposals show the very complex system of shower water and seal tank cascades required for countercurrent washing in a 5 or 6 stage conventional bleachery. In mill practice these flows are very difficult to keep in balance especially during periods of upset conditions which may occur quite frequently in normal mill operation. There have been proposals to not wash pulp at all during the bleaching process, although impurities were removed by thickening of the pulp after a low consistency first extraction stage. See in this regard, "The Bleaching of Pulp," Monograph No. 27, Chapter 17, pp. 346 et seq., Tappi (1963). This proposal resulted in low pulp viscosity and excessive and wasteful quantities of chemicals must be employed.
Others have eliminated shower water on some of the bleach washers, such as the D.sub.1 stage or E.sub.2 stage washers. See in this regard, Monograph No. 27, Chapter 17, supra; and Yankowski, Tappi, Vol. 55, No. 6, pp. 937-940 (June 1972). This has had the effect of eliminating the displacement wash component of the drum washer and reduced, but not necessarily eliminated, the dilution was obtained on the washer. The net effect has been to produce a smaller volume of more concentrated bleachery effluent for either internal or external treatment, but it does not provide the more rapid bleaching process of the present invention or the bleached pulp of enhanced properties and quality.
Before the proposals of countercurrent washing, when fresh water was used on the showers of all the washers, no attempt was made to adjust the flow of wash water with changes in production rate and there was no problem encountered with control of shower water to the washers. However, in a countercurrent washing system, problems are encountered keeping flows in balance, since the earlier bleach stages are dependent on availability of excess filtrate from later stage seal tanks for shower water. This problem of water balance is further aggravated when the production rate in some parts of the bleachery is different than in others, such as when the level in a down-flow bleach tower is lowered which increases production rate in the succeeding stages or vice-versa.
These problems of matching slower flow to production rate in countercurrent washing systems increase with increasing number of washers which must be kept in balance, i.e., increasing number of bleach stages with interstage washing and with the degree to which filtrates are recycled. The problem is greatest in a closed cycle mill using the Rapson-Reeve process, where all the water used in the bleachery has to be used in other parts of the kraft mill and ultimately all of the organic and inorganic waste products of bleaching enter the kraft liquor recovery system.
Other proposals have been made for eliminating washing in conjunction with one or more stages of a multistage bleaching process. Thus, U.S. Pat. No. 3,874,992 proposes press alkaline extraction pulp in which caustic is mixed rapidly with pulp at 10 percent consistency and then pressed to 30 to 40 percent consistency which effectively removed 2/3 to 3/4 of the liquid from the pulp before it was carried into a succeeding bleach stage. This process would not obtain the benefit of not washing before a chlorine dioxide stage (which is one of the advantages of the present invention) since most of the material that protects the cellulose from degradation would be removed in filtrate from the pressing. U.S. Pat. No. 2,587,064 suggests that in a bleaching sequence in which a chlorine dioxide stage (D) is to be followed by a hypochlorite stage (H), an intermediate washing between the two stages may be dispensed with, with a benefit to the brightness of the pulp. U.S. Pat. No. Re. 28,884 describes the omission of an intermediate water wash between a chlorine dioxide stage (D), employed as the initial bleaching stage, and a second stage which is a chlorination stage (C). This is known as sequential chlorination. U.S. Pat. No. Re. 28,887 has a disclosure similar to that of U.S. Pat. No. Re. 28,884, but suggests that washings may be eliminated after various stages of a multistage bleaching sequence, but not before a final chlorine dioxide (D) stage, so long as the first two stages are (1) chlorine dioxide followed by (2) chlorine.
The foregoing omissions of a washing stage in the bleaching sequence have not provided the advantages of the process of the present invention, which teaches the elimination of a washing step at particular stages in the multistage sequence.
In order to minimize the problems caused by the countercurrent washing processes and to obtain advantages over other processes which omit a washing step, it has been found possible by the process of the present invention to simplify the bleaching sequence so that there is a requirement of only three washing stages. In the simplified process of the invention, it is possible to produce a higher quality of pulp from a given bleach sequence; or, using fewer bleach stages, to produce a pulp quality that hitherto had only been possible by means of longer bleach sequences. For example, it has been possible to increase the pulp strength obtained from the C.sub.D EDED sequence by using three wash stages, instead of the conventional five wash stages, and it has been possible to produce a 90 GE brightness pulp from the C.sub.D EHD sequence with three wash stages which have the physical properties usually associated with the conventional C.sub.D EDED sequence while maintaining most of the fast beating characteristics generally associated with the hypochlorite stage. In addition, it is possible to shorten drastically the time of the intermediate stages of the sequence.
It is, therefore, an objective of the present invention to provide a bleaching process which requires the use of minimum amounts of water, time, capital expenditure, space requirements, and external heat, and produces pulp of consistently high quality in which the bleaching stages are conducted with conventional static flow of the pulp during retention periods so that there is no substantial movement of the bleaching liquid employed with respect to the fibers making up said pulp.
It is another objective of the present invention to provide a bleached pulp of greater strength than was formerly possible from a number of other bleach sequences.
It is another objective of the present invention to provide a bleaching process which reduces the number of bleaching stages required to produce a given quality of pulp.
Another objective of the present invention is to provide a bleaching system for a multistage bleaching process wherein the number of dilution and washing steps is minimized.
Another objective of the present invention is to provide a means which facilitates keeping the flows to bleachery washers in balance.
Another objective of the present invention is to minimize the opportunities for different production rates to occur in different parts of the bleachery at the same time, as is caused when the level changes in a conventional down-flow tower.